Two-piece plastic fasteners of the general type to which this invention relates are exemplified by the following U.S. Patents: U.S. Pat. No. 2,941,439, issued in 1960 to G. M. Rapata; U.S. Pat. No. 2,542,144, issued in 1951 to H. Kearns; U.S. Pat. No. 4,276,806, issued in 1981 to H. Morel; U.S. Pat. No. 3,417,438, issued in 1968 to J. T. Schuplin; and U.S. Pat. No. 3,385,158, issued in 1968 to L. H. Morin.
The prior patent that is probably best illustrative of the problems confronted and solved by the present invention is U.S. Pat. No. 3,918,130, issued to L. R. Poe in 1975. In general, the fastener of that patent comprises a grommet body having a head and a bifurcated shank that projects forwardly from the head and comprises a pair of legs that are laterally spaced from one another. After the grommet body has been inserted into aligned holes in a pair of workpieces that are to be fastened together, a plunger-like pin is driven forwardly through the head and between the legs to an operative position at which the front end portion of the pin holds the legs diverged to prevent the fastener from backing out of the workpieces.
In certain of its embodiments, the fastener of the Poe patent has a so-called web that is connected between the front ends of the legs of its grommet body. The web tapers forwardly to provide a reduced, coaxially conical front end portion on the shank of the grommet. The patent assigns to this web a function which relates to the structural integrity of the grommet body itself, but the web obviously has another and very desirable function when the fastener is inserted into aligned holes in a pair of workpieces that it is intended to connect, in that the web serves as a pilot that smoothly guides the grommet shank into the holes, even if the holes are not in good register with one another or the fastener is brought towards the holes in somewhat off-center relation to them. Without such a piloting web, it often happens that the front end of one leg of a grommet shank gets caught against a surface of the workpiece alongside the hole. Since the fastener tends to be installed with a fast and vigorous jab, such a hang-up may deform the fastener, but it will in any event necessitate some further manipulation of the fastener. For the installation of one fastener, such a fumbling and delay would be inconsequential; but an operator who works with plastic fasteners often installs hundreds or even thousands of them during the course of a shift, and therefore any feature that facilitates insertion is significant in terms of both economics and morale.
The fastener of the Poe patent had another desirable feature, in that its grommet body and pin could be molded in one piece, connected by a readily frangible flashing. Molding the two parts of a fastener in one piece was not broadly new with Poe, having been generally disclosed, for example, in U.S. Pat. No. 3,411,397, issued to D. W. Birmingham in 1968 and in the above mentioned Rapata Patent U.S. Pat. No. 2,941,439. The fasteners of the Birmingham and Rapata patents had to be molded with the use of an elongated core-like mold element that extended axially between the grommet legs and defined the slot between them, projecting rearwardly into the grommet head to the front end of the pin, to define a pin receiving bore in the grommet head. This elongated core element had to be withdrawn axially forwardly from between the legs after the fastener was molded, whereas mold parts that defined the external surfaces of the fastener had to move apart laterally.
Molding equipment comprising an axially forwardly movable core element was obviously unsuitable for producing the fastener of the Poe patent in its above described embodiment, because of the web connecting the front ends of the legs. Poe's fastener was therefore configured for molding in a single piece without the use of movable cores or the like, but to achieve this objective Poe arranged the head of the grommet body to comprise two complementary parts that defined spaced apart opposing surfaces. In the unit as molded, flashings that connected the pin to the grommet body extended only along the opposing surfaces of the head parts. In one embodiment of Poe's fastener the two parts of the head were connected by a thin web, but that web was present only at one side of the pin; at the other side of the pin the two head parts were unconnected, leaving a very large radially extending slot-like gap between the head parts that was continuous with the slot between the legs. Furthermore, the thin web that connected the two head parts was adjacent to the front surface of the head, leaving a deep groove in the grommet head, at its side remote from the legs, that appeared to be a continuation of the slot-like gap in the head at the other side of the pin.
Once the flashings were broken in Poe's fastener, the pin could be driven partway forwardly in the grommet to a detent-defined retracted position, but in that position, owing to the gap in the head, the pin had a substantial amount of side play in the grommet and could be rather easily removed from the grommet by lateral movement through the gap in the head. To prevent separation of the pin from the grommet during shipment and handling, commercial versions of the Poe fastener were delivered to the user with the flashings unbroken. This posed problems for the user. The pin, as it was driven forwardly in the grommet, received insufficient axial guidance, owing to the large radially extending gap in the head and the ease with which the head parts could be flexed apart to enlarge that gap; and consequently the pin could readily skew out of coaxial relation to the grommet.
Other problems grew out of the capability of the head parts for flexing convergence and divergence about the small web that connected them. Directly under the head of the pin was a lug-like projection that was intended to enter the gap between the head parts when the pin was driven all the way forward in the grommet, thereby preventing convergence of the head portion of the grommet in response to forces that tended to produce relative shifting between workpieces connected by the fastener. However, if a fastener hole in a workpiece was slightly undersize, the grommet could be inserted easily enough into the hole, but the head parts would be held converged, narrowing the gap between them to block entry into that gap of the lug-like projection on the pin. Under these conditions, the pin could not be driven all the way forward to its operative position without the use of enough force to damage head portions of the grommet and the pin.
These problems with the fastener of the Poe patent were all due to the configuration of the grommet head, insofar as it was essentially formed in two distinct parts that were connected--if at all--by only a small web. (In one embodiment of the Poe fastener the parts of the head were connected with one another only very indirectly, by means of the web at the front ends of the legs.) In turn, the reason for forming the grommet head in two substantially separate parts was to enable production of the fastener by means of molding equipment that comprised no axially movable mold elements. The gap in the head allowed a laterally movable male mold element to define the front end surface of the pin and to core the aperture in the grommet head through which the pin moved forward as well as the slot between the legs of the grommet.
The fastener of the Poe patent has advantages--particularly with respect to low production costs--that have earned for it a certain amount of commercial success. On the other hand, such commercial success as it has enjoyed has clearly demonstrated and emphasized the problems that it presents, which impose rather stringent limits upon the applications for which it can be employed. Evidently it has not heretofore been obvious to those skilled in the art how the manifest deficiencies in the Poe fastener can be remedied without sacrifice of its clearly desirable features, and particularly its front web that serves as a pilot for its grommet shank and its capability for production by means of molding apparatus having mold parts that move only in a pair of opposite directions.